The body temperature of marine teleosts living in polar waters is in temperature equilibrium with the surrounding sea, and is thus at a temperature of approximately −1.8° C. during winter or year round in for example Antarctic waters. The blood of marine teleosts is hypoosmotic to the seawater and its melting point is predicted to be approximately −0.7° C. The polar teleosts are thus supercooled and lethal freezing would be expected in the absence of an adaptation to such harsh climatic conditions.
Similar observations are applicable for a variety of cold-adapted terrestrial organisms, including arthropods, plants, fungi and bacteria although many of these organisms are subjected to much lower temperatures than marine fish. Psychrophilic (cold-loving) organisms have successfully adapted to all the permanently cold regions on earth: They can be found in the deep sea, on freezing mountain tops and in the Polar regions even at temperatures as low as −60° C. Despite the lethal conditions, these organisms have overcome key barriers inherent to permanently cold environments. These barriers include: reduced enzyme activity, decreased membrane fluidity, altered transport of nutrients and waste products, decreased rates of transcription, translation and cell division, polypeptide cold-denaturation, inappropriate polypeptide folding and intracellular ice formation.
Research on the mechanisms that allow certain organisms to exist at subzero temperatures has revealed that they rely on at least two strategies: Lowering of the freezing point of water (colligatively by synthesis of low molecular weight substances and non-colligatively via synthesis of unique polypeptides) by either inhibiting ice growth or by giving rise to controlled ice crystal formation. Anti-freeze polypeptides (AFPs) and low molecular weight substances, such as polyalcohols, free amino acids and sugars are believed to be responsible for the former process, while Ice Nucleating polypeptides (INPs) are responsible for the latter.
Anti-freeze polypeptides (AFP—in some publications also known as thermal hysteresis polypeptides, THP, or ice structuring polypeptides, ISP) lower the freezing point of a solution substantially while the predicted melting point is only moderately depressed. This means that whereas the freezing point is lowered dramatically, the melting point of the solution is predicted by the colligative melting point depression. This is true for solutions where ice is present—the question as to whether anti-freeze polypeptides can lower the supercooling point of an ice-free solution is largely unsolved.
The displacement of the freezing temperature is limited and rapid ice growth will take place at a sufficiently low temperature. The separation of the melting and freezing temperature is usually referred to as thermal hysteresis (TH) (Knight et al. 1991, Raymond and DeVries 1977, Wilson 1993), and the temperature of ice growth is referred to as the hysteresis freezing point. The difference between the melting point and the hysteresis freezing point is called the hystersis or the anti-freeze activity. A second functionality of the AFPs is in the frozen state, where they show ice recrystallization inhibition (RI). The AFPs inhibit the formation of large crystals at the expense of small crystals at temperatures above the temperature of recrystrallisation. (Knight et I. 1984, 1995, Knight and Duman 1986, Ramlov et al. 1996).